There is currently a large and growing requirement for NF3 in semiconductor manufacturing. One of the early processes for producing NF3 involves the direct fluorination of ammonium ions by F2 whereby gaseous F2 is contacted with liquid (molten) ammonium acid fluoride (AAF) while gaseous NH3 is separately contacted with the liquid AAF to generate ammonium ions. The early processes are operated to maintain a molar ratio of by-product HF to ammonia of 2.0 to 2.5 (melt ratio) in the reaction liquid and at temperatures above the melting point of ammonium bifluoride, NH4HF2, which is 127° C. Later processes for producing NF3 effect the direct fluorination of ammonium bifluoride using increased HF/NH3 melt ratios. These later processes resulted in superior NF3 yields than when the lower melt ratio processes were used.
Purification of the NF3 reaction product formed by the direct fluorination of ammonium bifluoride involves the removal of unreacted or unconverted F2, as well as the byproducts HF, N2, and trace levels of N2F2, and N2O. Conventional practice in prior purification processes effects the removal of excess F2 and co-product HF by aqueous KOH scrubbing followed by removal of trace levels of nitrogen oxides and water present in the stream using a solid molecular sieve adsorbent. Finally, the NF3 is further purified using distillation.
The following patents and articles describe processes for production and purification of NF3.
U.S. Pat. No. 4,091,081 discloses a process for the production of NF3 by the direct fluorination of ammonium bifluoride at temperatures above 260° F. and below 400° F. An HF/NH3 ratio of 2 to 2.5 is maintained. Purification is effected by passing the reaction product through a mist eliminator pad and then through an aqueous KOH scrubber. Residual HF and unreacted F2 are removed in the scrubber. After scrubbing, the temperature of the stream is reduced to condense water and then the stream passed through molecular sieve driers for further purification.
U.S. Pat. No. 4,156,598 describes a process for the production NF3 by the direct fluorination of ammonium bifluoride. Purification of the gaseous reaction stream involves passing the gaseous reaction stream through a mist eliminator pad to remove entrained ammonium fluoride or bifluoride and then through a bath capable of forming a salt of HF and F2, e.g., an aqueous KOH bath. Alternatively, sodium fluoride is used in place of KOH, but F2 is not removed by this method. To extend the lifetime of the adsorber columns N2F2 is removed to a level below 0.03 volume percent before adsorption of N2O and water.
U.S. Pat. No. 4,543,242 describes a process for the production and purification of NF3. NF3 is purified by removing HF by condensation. Further purification of NF3 is accomplished by wet scrubbing of NF3 using aqueous KOH followed by treatment using a molecular sieve.
U.S. Pat. No. 4,933,158 discloses a process for the purification of NF3 produced by various methods, including the direct fluorination of ammonium bifluoride. Removal of N2O, CO2, and N2F2 from an NF3 stream is accomplished by passing the stream through a thermally treated natural zeolite.
U.S. Pat. No. 4,980,144 describes a process for purification of NF3 containing hydrogen fluoride, and oxygen difluoride. In the described process, NF3 is generated under conditions such that OF2 is generated in amounts in excess of 700 ppm. Typically, the NF3 is generated by a process referred to as molten salt electrolysis wherein a salt comprised of ammonium fluoride and HF are electrolyzed. In the NF3 purification process, HF is removed first by contact with water or a caustic scrubber. Then, the OF2 present in the stream is removed to a level below about 10 ppm by contacting the gas with an aqueous solution of sodium thiosulfate, hydrogen iodide, and sodium sulfide. The NF3 stream is then passed through a zeolite molecular sieve to remove water.
U.S. Pat. No. 5,637,285 discloses the production or NF3 by the direct fluorination of ammonium bifluoride at a melt ratio of HF/NH3 of at least 2.55.